The present invention relates to a wavelength division multiplexed optical transmission system which transmits a wavelength division multiplexed optical signal using a dispersion-shifted fiber.
Wavelength Division Multiplexing (WDM) transmission technology is a technology in which optical signals of differing wavelength (optical frequency) are multiplexed, and transmitted via one optical fiber transmission path. Here, the optical signal is the optical output of a light source directly modulated by a data signal (direct modulation type), or an optical transmission wave output from a light source modulated by a data signal using an external modulator (external modulation type), and this wavelength is determined by the light source wavelength.
By disposing along the optical fiber propagation path optical amplifiers which amplify the optical signal as-is, and compensating the transmission loss of the optical fiber transmission path, it is possible to extend the span between regenerative repeaters which are necessary for discriminative reproduction processing at the electrical step. This optical amplifier can increase the transmission capacity of an installed optical fiber transmission path by many times the number of wavelengths simply by altering the transmission and receiving apparatuses for wavelength division multiplexing use because it possesses a function in which optical signals of differing wavelength are amplified together. For example, the amplification wavelength bandwidth of an erbium doped optical fiber amplifier (EDFA) is between 1.53 xcexcm and 1.56 xcexcm, and by multiplexing optical signals at wavelength intervals of 0.8 nm in this wavelength band, about 30 channels of optical signals can be transmitted through in one optical fiber.
However, installed dispersion-shifted fibers transmit optical signals of a designed zero-dispersion wavelength. When transmitting wavelength division multiplexed optical signals in this dispersion-shifted fiber, cross-talk due to four-wave mixing, a non-linear optical effect, is generated, and because of this the input power to the transmission path fiber could not be increased. In the following this problem will be explained in detail.
The propagation loss of a silica optical fiber is minimal in the 1.5 xcexcm to 1.6 xcexcm region. A dispersion-shifted fiber is designed so that the wavelength dispersion is zero in the 1.55 xcexcm wavelength region, and by suppressing waveform degradation due to wavelength dispersion at this wavelength, the transmission distance can be increased. In addition, while the International Standards Organization has stipulated that the zero dispersion wavelength of a dispersion-shifted fiber is allocated between 1.525 xcexcm and 1.575 xcexcm, practically the distribution is roughly between 1.535 xcexcm and 1.565 xcexcm, centered on 1.550 xcexcm, and up to the present, these have been widely installed.
In contrast, when optical signals of differing optical frequencies are input into an optical fiber, new optical frequencies dependent on the difference in input optical frequencies are generated based on third-order non-linearity within the optical fiber. This is called xe2x80x9cfour-wave mixing,xe2x80x9d and is a phenomenon wherein, for example, an optical frequency f1+f2xe2x88x92f3 is generated from input optical frequencies f1, f2, and f3. This four-wave mixing is more easily generated the smaller the dispersion value of the input optical wavelength, or the larger the input power of each individual wavelength.
If the optical frequency intervals between the wavelength division multiplexed optical signals input into this kind of optical fiber are uniform, the optical frequency newly produced by four-wave mixing will conform with one optical wavelength among those of the optical signal, and strong noise will be generated by mutual interference. In addition, even when the optical frequency intervals of the wavelength division multiplexed optical signal are not uniform, the optical power of the original optical signal is consumed in the generation of four-wave mixing, and this produces strong noise; When the optical frequency interval of the wavelength division multiplexed optical signal has even spacing, excess noise originating in four-wave mixing is generated by an input power per wavelength from about xe2x88x925 dBm, and when the spacing is uneven, it is generated by an input power per wavelength from about xe2x88x922 dBm. Because of this, the optical power that can be input into the optical fiber transmission path cannot exceed this value, and as a result, the transmission distance is limited.
The object of the present invention is to provide a wavelength division multiplexed optical transmission system which can use dispersion-shifted fibers installed in an optical transmission path and transmitting wavelength division multiplexed optical signals, and can increase the permissible optical input power to a dispersion-shifted fiber.
The present invention is a wavelength division multiplexed optical transmission system or a wavelength division multiplexed optical transmission method wherein among wavelength division multiplexed optical signals the wavelengths of either of at least two optical signals are between 1450 nm and 1530 nm, or between 1570 nm and 1650 nm when a dispersion-shifted fiber whose zero dispersion wavelength is in the 1550 nm region is used as a transmission path.
In this manner, by limiting the used wavelength bandwidth, the influence of four-wave mixing in the dispersion-shifted fiber can be avoided. Thus, it is possible to enlarge the permissible input power to the dispersion-shifted fiber.